binman: Adjust pylibfdt for incremental build

binman: Adjust pylibfdt for incremental build

If the pylibfdt shared-object file is detected, then Python assumes that
the libfdt.py file exists also.

Sometimes when an incremental build aborts, the shared-object file is
built but the libfdt.py is not. The only way out at this point is to use
'make mkproper', or similar.

Fix this by removing the .so file before it is built. This seems to make
Python rebuild everything.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>